Although refineries produce many products, the most desirable are the transportation fuels gasolines, diesel fuels, and jet fuels, as well as light heating oils, all of which are high-volume, high value products. While light heating oils are not transportation fuels, their hydrocarbon components are interchangeable with diesel and jet fuels, differing primarily in their additives. Thus, it is a major objective of petroleum refineries to convert as much of the barrel of crude oil into transportation fuels as is economically practical. The quality of crude oils is expected to slowly worsen with sulfur and metals content and densities increasing. Greater densities mean that more of the crude oil will boil above about 560.degree. C., and thus will contain higher levels of Conradson Carbon and/or metal components. Historically, this high-boiling material, or residua, has been used as heavy fuel oil, but the demand for these heavy fuel oils has been decreasing because of stricter environmental requirements. This places greater emphasis on refineries to process the entire barrel of crude to more valuable lower boiling products.
Coking processes are presently the major refinery processes for converting heavy feeds, such as residua, to more valuable lower boiling products, but are typically too severe for obtaining optimum mounts of gasoline and distillate boiling products without producing an undesirable amount of coke and light gases. It would be desirable to first distill, or vaporize, volatile materials of resids prior to coking to obtain higher yields of such desirable transportation fuel products.
The two types of coking most commonly commercially practiced are delayed coking and fluidized bed coking. In delayed coking, the resid is heated in a furnace and passed to large drums maintained at temperatures from about 415.degree. C. to 540.degree. C. During a long residence time in the drum at such temperatures, the resid is converted to coke. Liquid products are taken off the top for recovery as "coker gasoline", "coker gas oil", and gas. Conventional fluidized bed coking process units typically include a coking reactor and a burner. A petroleum feedstock is introduced into the coking reactor containing a fluidized bed of hot solids, preferably coke, and is distributed uniformly over the surfaces of said coke particles where it is cracked to vapors and to carbonaceous material which is deposited onto the particles. The vapors pass through cyclones which remove most of the entrained coke particles. The vapor is then discharged into a scrubbing zone where remaining coke particles are removed and the products are cooled to condense heavy liquids. The resulting slurry, which usually contains from about 1 to about 3 wt. % coke particles, is recycled to extinction to the coking zone.
The coke particles in the coking zone flow downwardly to a stripping zone at the base of the coking reactor where a stripping gas, such as steam, is used to remove interstitial product vapors from, or between, the coke particles, as well as some adsorbed liquids from the coke particles. The coke particles then flow down a stand-pipe and into a riser which moves them to a burner where sufficient air is injected for burning at least a portion of the coke and heating the remainder sufficiently to satisfy the heat requirements of the coking zone where the unburned hot coke is recycled. Net coke, above that consumed in the burner, is withdrawn as product coke.
While fluidized bed coking has met with a substantial amount of commercial success, there still remains a need in the industry for methods that can increase the liquid yields, the quality of liquids, or both.